Wells Fargo, the controversy-battered big bank, has a new problem — this one directly affecting mortgage applicants. Last week, a first-time home buyer filed a class-action suit against the company, alleging widespread abuse of a procedure well known to most mortgage borrowers: interest-rate locks.

The suit alleges that Wells Fargo engaged in “a systematic effort” to charge unwarranted rate-lock extension fees — sometimes costing thousands of dollars each — to borrowers who should not have been required to pay them. The Consumer Financial Protection Bureau is investigating the same practices, according to Wells Fargo’s most recent quarterly filing with the Securities and Exchange Commission.